Spinach (Spinacia oleracea) is a flowering plant from the Amaranthaceae family that is grown as a vegetable. The consumable parts of spinach are the leaves from the vegetative stage. Spinach is sold loose, bunched, in prepacked bags, canned, or frozen. There are three basic types of spinach, namely the savoy, semi-savoy and smooth types. Savoy has crinkly and curly leaves. Flat or smooth leaf spinach has in general broad, smooth leaves. Semi-savoy is a variety with slightly crinkled leaves. The main market for spinach is baby-leaf. Baby spinach leaves are usually of the flat-leaf variety and usually the harvested leaves are not longer than about eight centimeter. These tender, sweet leaves are sold loose rather than in bunch. They are often used in salads, but can also be lightly cooked. Downy mildew—in spinach caused by the pathogen Peronospora farinosa f. sp. spinaciae (formerly known as P. effusa)—is a major threat for spinach growers, because it affects the harvested plant parts, namely the leaves. Infection makes the leaves unsuitable for sale and consumption, as it manifests itself phenotypically as yellow lesions on the older leaves, and on the abaxial leaf surface a greyish fungal growth can be observed. The infection can spread very rapidly, and it can occur both in glasshouse cultivation and in soil cultivation. The optimal temperature for formation and germination of P. farinosa f. sp. spinaciae is 9 to 12° C., and it is facilitated by a high relative humidity. When pathogens are deposited on a humid leaf surface they can readily germinate and infect the leaf. Pathogen growth is optimal between 8 and 20° C. and a relative humidity of ≥80%, and within 6 and 13 days after infection growth can be observed. Pathogens of P. farinosa can survive in the soil for up to 3 years, or in seeds or living plants. In recent years various resistance genes have been identified that provide spinach plants with a resistance against downy mildew. However, it has been observed that previously resistant spinach cultivars can again become susceptible to the pathogen. Investigations revealed that the cultivars themselves had not changed, and that the loss of downy mildew resistance must therefore be due to P. farinosa overcoming the resistance in these spinach cultivars. The downy mildew races that were able to infect resistant spinach cultivars have been determined on a differential reference set, used to test spinach cultivars for resistance. The differential set comprises a series of spinach cultivars (hybrids) that have different resistance patterns to the currently identified pathogenic races. To date 15 pathogenic races of spinach downy mildew (Pfs) have been officially identified and characterized. Races 4 through 10 have been identified between 1990 and 2009, which illustrates the versatility and adaptability of the pathogen to overcome resistances in spinach. In 2014, isolate UA1014APLP (also known as UA1014) was identified by the Correll lab of Arkansas.
In different geographical regions different combinations of pathogenic races or isolates occur, and the spinach industry therefore has a strong demand for spinach cultivars that are resistant to as many relevant downy mildew races as possible, preferably to all races that may occur in their region, and even to the newest threats that cannot be countered with the resistances that are present in the commercially available spinach cultivars.
It is crucial to stay at the forefront of developments in this field, as Peronospora continuously develops the ability to break the resistances that are present in commercial spinach varieties.
For this reason new resistance genes are very valuable assets, and they form an important research focus in spinach breeding. The goal of spinach breeders is to rapidly develop spinach varieties with a resistance to as many Peronospora races as possible, including the latest identified. To date, 15 Pfs races are officially recognized and made publicly available from the Department of Plant Pathology, University of Arkansas, Fayetteville, Ark. 72701, USA, and also from NAK Tuinbouw, Sotaweg 22, 2371 GD Roelofarendsveen, the Netherlands. Pfs 15 was officially recognized in 2014 by the International Working Group on Peronospora farinosa (IWGP). Recently identified isolate UA1014APLP is yet to be officially recognized as a race, but is to date already of importance to breeders.
It is the object of the invention to provide a spinach plant, conferring resistance to various Peronospora races and/or isolates, including the ones that have been most recently identified.